A thin oxide film formed on a film-forming metal or valve metal such as aluminum and tantalum by anodic oxidation of the metal serves as a dielectric of a solid capacitor. As is well known in the art, such a film possesses essentially eminent dielectric characteristics, but practically it cannot be devoid of considerable faults and micropores developed during and/or after anodization. As a result, dielectric characteristics and leakage current of a capacitor utilizing an anodized oxide film always remain below the levels expected from the values inherent to an ideal oxide film. An electrolyte disposed between the dielectric oxide film and a counter electrode of a practical electrolyte capacitor, therefore, is required to have a function of electrolytically oxidizing the anode in the case of polarization thereof to reform the defective oxide film apart from its principal function as a substantial cathode.
Manganese dioxide is the most familiar solid electrolyte now in practical use. According to generally accepted explanations for the function of manganese dioxide to reduce the leakage current of a capacitor, faults in the dielectric oxide film may be healed by oxygen liberated from polarized manganese dioxide, and/or manganese dioxide may be reduced to a lower and non-conducting oxide due to high temperatures produced by high density of currents through the faults.
There is, however, a significant problem in forming a manganese dioxide coating on an anodized valve metal, viz., a thermal treatment at considerably high temperatures is required. For example, thermal decomposition of manganese nitrate to the dioxide is usually carried out at temperatures between 200.degree. and 400.degree.C. Besides, application of the nitrate solution and thermal decomposition thereof must be repeated several times in order to form a dense and closely adhered manganese dioxide coating. The multiple exposure to the high temperatures inevitably damages the inherently feeble dielectric oxide film and results in unsatisfactory characteristics of the produced capacitor. Accordingly it is necessary to provide repeated steps of re-anodizing between and after the heating steps. A capacitor produced by such complicated procedures has nevertheless shortcomings such as a relatively large leakage current and a relatively low maximum operating voltage.
Various organic semiconductive substances have been proposed to replace manganese dioxide and thereby to eliminate the above drawbacks. Examples of organic semiconductive substances promising as solid electrolyte material for their good anodical oxidizing prroperties are some charge transfer complex compounds the acceptor of which is a nitro compound or a quinolinium compound. Another group of organic semiconductive compounds featuring superior anodizing capability is a group of salts having 7,7,8,8-tetracyanoquinodimethane, hereinafter referred to as TCNQ for brevity, as the anion component. Solid electrolyte capacitors based on these organic semiconductive compounds are described, e.g. in U.S. Pat. No. 3,586,923. These compounds can be coated on an anodic oxide film without requiring high temperatures and hence scarcely damage the oxide film and exhibit better anodizing or reforming properties than manganese dioxide.
It is now beyond doubt that organic semiconductive compounds of the charge transfer complex type and especially some salts of TCNQ serve, at least theoretically, as advantageous electrolytes for solid electrolyte capacitors, but improved capacitors of practical use can be obtained only when a practical method of coating such a compound on the dielectric oxide film is established. The method is required to give a dense, uniform and strongly adhering coating without injuring the inherent property of the compound or its ability to reform an anodic oxide film.
At first, application of an organic solvent solution of a TCNQ salt was proposed, e.g., by U.S. Pat. Nos. 3,214,648 and 3,214,650. Although the method is quite easy to carry out, such a method usually fails in providing the desired degree of density and adhesion strength of the resulting coating. Next, a semiconductive polymer or a polymer capable of dissolving a semiconductive salt has been sought, and U.S. Pat. Nos. 3,424,698 and 3,483,438 disclose some polymers which dissolve TCNQ and its salts and electrolyte capacitors based on such polymers, respectively. A problem with respect to the polymers according to these patents resides in that relatively large amounts of the polymer are required to obtain an electrolyte coating of such denseness and adhesion as to ensure the desired level of stability and life of the resulting capacitor. In the capacitors of U.S. Pat. No. 3,483,438, for example, a polymer content of far more than 50% by weight is necessary to prepare an electrolyte system in which a TCNQ salt is dissolved up to saturation.
The smaller the amount of polymer binder in an electrolyte system, the better for obtaining a capacitor of excellent characteristics, because the capacitor characteristics are substantially determined by the physical properties of the polymer itself when a large amount of polymer is used. Practical disadvantages resulting from a large polymer content in the electrolyte layer of a solid capacitor are as follows:
1. increase in the specific resistance of the electrolyte and hence increase in the dielectric loss of the capacitor;
2. decrease in the capacitance of the capacitor;
3. deterioration in the anodizing property and dielectric strength; and
4. deterioration of the capacitor due to expansion of the polymer. These disadvantages may be reduced to permissible levels if the polymer content is limited to 40% by weight at most.